This project is concerned with the role of platelets in recurrent venous thromboembolism and in the embolism of patients with rheumatic heart disease. In recurrent venous thrombosis platelet survival time is abnormally shortened in most patients, and the platelet suppressant, sulfinpyrazone, lengthens platelet survival. Fibrinogen survival and blood fibrinolytic activity is less frequently abnormal and not altered by sulfinpyrazone. Shortened platelet survival correlates with a history of systemic embolism in patients with rheumatic heart disease, and sulfinpyrazone lengthens platelet survival in these patients. The study will enroll patients with rheumatic heart disease in a prospective trial of sulfinpyrazone. Yearly measurements of platelet survival will be performed and patients with shortened platelet will be randomized to either sulfinpyrazone or placebo. One hundred and one patients have been enrolled as of April, 1976.